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Impact Of Cya On "real" Fc?


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I've been doing too much reading on water chemistry, and having trouble getting a grip on what CYA does to the actual levels of chlorine available for sanitizing my tub. If I understand what I read in ChemGeek's (excellent) articles at TroubleFreePool correctly, I need to keep a base level of FC equal to 10% of my CYA level to ensure there is always some chlorine available for sanitation.

Am I understanding that right?

Background: My tub is a 2011 HotSpring Vanguard with ozone. I'm using the Dichlor then Bleach method, and changed the water 3 weeks ago. My routine is to add enough bleach each afternoon to bring my tub to 6ppm FC. (I'm in the tub for 20-25 minutes every night, my wife may join me 2-3 times/week). I usually see 1-2ppm FC when I test before adding the bleach. Given what I've read about CYA, am I actually leaving my tub with NO effective sanitation for part of the day? I just tested CYA, and it's somewhere between 30 and 40.

This stuff is making my head spin just a bit. The funny thing is that my wife is a high school chemistry teacher, and wants absolutely nothing to do with this!

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It's not an absolute cutoff as you are implying. It's proportional. The amount of active chlorine (hypochlorous acid) is proportional to the FC/CYA ratio. An FC that is 10% of the CYA level is a rough rule-of-thumb mostly set at the level to prevent algae growth in pools (the actual minimum in the chlorine/CYA charts is more like 7.5% for non-SWG manually dosed pools and 5% for SWG pools). It is also a level that kills most pathogens quickly (fecal bacteria 99.9% killed in less than 1 minute). So your active chlorine does not go to zero just because your FC level got below 10% of the CYA level. Don't worry about that.

Also, for hot tubs, it's easier since you just keep your CYA level around 30 ppm, though anything in the 20-50 ppm should be OK, and you just make sure your FC level is not lower than around 1-2 ppm FC when you start your soak which, by definition, will mean that it wasn't at zero at anytime since after your last soak when you last added chlorine.

If you are going to be soaking in a situation where you wanted to ensure fast-acting sanitation during the entire time of the soak, then you'd need a higher FC level such that the FC was 1-2 ppm FC at the end of your soak -- so probably would need to start with around 4 ppm or so depending on your bather load. Most people don't like to do that since the chlorine byproducts will more noticeably smell in that case. The risks when having low chlorine in a residential spa during the soak are low because 1) soak time is usually one hour or less so bacteria cannot multiply very much in that time, 2) there is usually monochloramine present that will still kill bacteria though more slowly and 3) you are usually soaking alone or with your family that is unlikely to have diarrhea or serious illness. Chlorine is used for disinfection primarily to prevent runaway bacterial growth (which would mostly be in the time in between soaks) and to prevent person-to-person transmission of disease (which would be during a soak) though the latter is much more important in a public/commercial pool where one person can infect many and where there is far less control over keeping sick people out of the water.

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